Simplify and correct tipsy sub-chunking.

[matthewturk]

Fixes #2276.

The tipsy chunking was not correctly subchunking, and this seemed to only show up when the particular ordering and sizes of the particle types was set in a specific way. This would always have a "hard fail", so there have to my knowledge this has not been the source of any quiet failures.

What this does is simplify the particle offset calculation. Instead of our previous way of doing things, which I could not quite parse out (and which was probably dating from before we did any subchunking) the new way is to compute a global offset for each particle type and then offset into that based on data_file.start. I also terminate earlier when trying to read particles that aren't in a sub-chunk.

This uses data from @claytonstrawn .

My verification script, which I ran with changing CHUNKSIZE in yt/geometry/particle_geometry_handler.py to a big number and leaving it at the small number:

import yt
import os

if os.path.isfile("testnihaoinput/g8.26e11/g8.26e11.00320.ewah"):
    print("removing ewah")
    os.unlink("testnihaoinput/g8.26e11/g8.26e11.00320.ewah")
if os.path.isfile("testnihaoinput/g8.26e11/g8.26e11.00320-hsml"):
    print("removing hsml")
    os.unlink("testnihaoinput/g8.26e11/g8.26e11.00320-hsml")

#ds = yt.load("testnihaoinput/g2.63e10/g2.63e10.00320")
ds = yt.load("testnihaoinput/g8.26e11/g8.26e11.00320")

dd = ds.all_data()

import h5py

f = h5py.File("output_big.h5", "w")

for ft, fn in ds.field_list:
    if ft not in f: f.create_group(ft)
    print("Creating ", ft, fn, dd[ft, fn].size)
    f[ft].create_dataset(fn, data=dd[ft, fn])
f.close()

The results are identical between the two. I should note that for some reason reading "smoothing_length" doesn't work, which is a bit confusing for me.

[brittonsmith]

This line is triggering the flake8 error since params is no longer used.

[matthewturk]

Should be fixed now!